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United States Patent |
6,040,683
|
Mottier
|
March 21, 2000
|
Battery charger with active feedback voltage controller
Abstract
A battery charger (100) with active feedback voltage controller includes a
pass device (144), such as a metal-oxide-semiconductor field-effect
transistor (MOSFET), and uses either hardware, software, or a combination
of hardware and software, to implement an active feedback voltage
controller (130). The active feedback voltage controller (130) reduces the
control voltage to a tracking regulator (110) which in turn reduces the
power dissipation of the pass device (144) and allows for a smaller pass
device to be implemented in the battery charger (100) while maintaining
the desired charging current as determined by a current controller (120).
With software flexibility, many types of batteries can be efficiently
charged to capacity, including nickel-cadmium (NiCad),
nickel-metal-hydride (NiMH), and lithium-ion (LiIon) batteries.
Inventors:
|
Mottier; Matthew D. (Lake Zurich, IL)
|
Assignee:
|
Motorola, Inc. (Schaumburg, IL)
|
Appl. No.:
|
322093 |
Filed:
|
May 28, 1999 |
Current U.S. Class: |
320/137 |
Intern'l Class: |
H02J 007/04; H02J 007/16 |
Field of Search: |
320/128,132,134,136,137,150,152,162,163,164
|
References Cited
U.S. Patent Documents
5130634 | Jul., 1992 | Kasai.
| |
5481174 | Jan., 1996 | Martin et al.
| |
5498950 | Mar., 1996 | Ouwerkerk.
| |
5587649 | Dec., 1996 | Garrett.
| |
5691620 | Nov., 1997 | Nagai et al.
| |
5703470 | Dec., 1997 | Baranowski et al.
| |
5818199 | Oct., 1998 | Beard.
| |
Primary Examiner: Tso; Edward H.
Attorney, Agent or Firm: Chen; Sylvia
Claims
I claim:
1. A battery charger with active feedback voltage controller comprising:
a pass device configured for connection between a tracking regulator and a
battery pack;
a current controller, coupled to the pass device, for determining a present
charging current going to the battery pack and determining a current
control value for controlling the pass device based on the present
charging current; and
an active feedback voltage controller, coupled to the current controller
and configured for connection to the tracking regulator and the battery
pack, for receiving the current control value and an actual battery
voltage and determining a voltage control value to control an output
voltage of the tracking regulator based on the current control value and
an actual battery voltage.
2. A battery charger according to claim 1 wherein the active feedback
voltage controller comprises:
a hardware feedback loop.
3. A battery charger according to claim 2 wherein the active feedback
voltage controller further comprises:
an PNP bipolar junction transistor.
4. A battery charger according to claim 3 wherein a base of the PNP bipolar
junction transistor is coupled to receive a voltage representing the
current control value, an emitter of the PNP bipolar junction transistor
is coupled to the battery pack, and a collector of the PNP bipolar
junction transistor is coupled to the tracking regulator.
5. A battery charger according to claim 2 wherein the active feedback
voltage controller further comprises:
an NPN bipolar junction transistor.
6. A battery charger according to claim 5 wherein a base of the NPN bipolar
junction transistor is coupled to receive a voltage representing the
current control value, an emitter of the NPN bipolar junction transistor
is coupled to ground, and a collector of the NPN bipolar junction
transistor is coupled to the battery pack and the tracking regulator.
7. A battery charger according to claim 1 wherein the active feedback
voltage controller comprises:
a software feedback loop.
8. A battery charger according to claim 1 wherein the current controller
comprises:
a temperature sense input configured for connection to a thermistor.
9. A battery charger according to claim 1 wherein the current controller
comprises:
a data input configured for connection to a data storage device.
10. A battery charger with active feedback voltage controller comprising:
a pass device having an input configured for connection to a tracking
regulator and an output configured for connection to a battery pack; and
a controller, coupled to the pass device, having:
a means for determining a present charging current going to the battery
pack and determining a current control value;
a current control line, coupled to a gate of the pass device, for using the
current control value to control the pass device;
a battery line, coupled to the battery pack, for sensing an actual battery
voltage; and
a voltage control feedback line, configured for connection to the battery
pack and the tracking regulator, for controlling an output voltage of the
tracking regulator based on the current control value and the actual
battery voltage.
11. A battery charger with active feedback voltage controller according to
claim 10 further comprising:
a sense resistor having a resistance value, coupled between the tracking
regulator and the pass device, wherein the means for determining a present
charging current calculates the present charging current by dividing a
voltage drop across the sense resistor by the resistance value.
12. A battery charger with active feedback voltage controller according to
claim 10 wherein the controller further comprises:
a temperature sense input for connection to a thermistor.
13. A battery charger with active feedback voltage controller according to
claim 10 wherein the controller further comprises:
a data input for connection to a data storage device.
14. A method for charging a battery comprising the steps of:
coupling a tracking regulator to a pass device;
coupling the pass device to the battery;
determining a present charging current going to the battery;
computing a current control value;
sensing an actual battery voltage of the battery;
increasing a voltage on a voltage control feedback line relative to the
actual battery voltage when the present charging current is less than the
current control value;
decreasing a voltage on the voltage control feedback line relative to the
actual battery voltage when the present charging current is greater than
the current control value; and
decreasing a voltage on the voltage control feedback line relative to the
actual battery voltage when the present charging current is approximately
equal to the current control value.
Description
FIELD OF THE INVENTION
This invention relates generally to battery chargers, and more particularly
to limiting power dissipation in battery chargers that use a pass device.
BACKGROUND OF THE INVENTION
Portable electronic devices, such as radiotelephones, currently use
batteries as their main power source. Adapters, such as hands-free
adapters, mobile transceiver adapters, cigarette lighter adapters, or wall
charger adapters, can be connected to a vehicle cigarette lighter or an
electrical outlet to provide an external power source for charging a
battery attached to the portable electronic device. Many of these portable
electronic devices use internal battery chargers to decrease the size of
the adapters and increase convenience to the user.
In one type of internal battery charger, called a series pass charger, a
linear switch pass device such as a metal-oxide-semiconductor field-effect
transistor (MOSFET) is connected between a regulator and the battery. When
a battery is charging, the power dissipated by the pass device is equal to
the difference between the input and output voltages of the pass device
multiplied by the maximum charging current. When a battery is deeply
discharged, the battery voltage, which is the voltage at the output of the
pass device, is much less than the regulator voltage, which is the voltage
at the input of the pass device. During this condition, the power
dissipated by the pass device could exceed maximum power ratings of
typical device packages found in portable electronic devices. During a
period of high power dissipation by the pass device, excess heat is
generated and the overall efficiency of the battery charger is very poor.
Some internal battery chargers use an external tracking regulator
physically located in the adapter to limit power dissipation in the
charger's pass device. The tracking regulator provides a voltage that is a
constant positive offset from the voltage of the battery being charged,
thus holding the difference between the input and output voltages of the
pass device relatively constant. When charging the battery, a
microprocessor creates a control voltage proportional to a desired
charging current, which controls the pass device (e.g., the gate of the
MOSFET). The actual charging current is measured by a feedback loop that
senses a voltage drop across a sense resistor, scales it, and compares it
to the control voltage.
Even the use of an external tracking regulator, however, does not
sufficiently reduce the power dissipation of an internal battery charger
under certain conditions. For example, when the internal battery charger
is implemented in a very small radiotelephone, the package of the pass
device may be too small to properly dissipate the heat created by the pass
device. Using a larger pass device package would make heat dissipation
more efficient, but the drawback is that a larger package would make it
difficult to fit the internal battery charger into the very small
radiotelephone. Thus, it would be advantageous to further limit power
dissipation in the internal battery charger to allow a reduction in the
size of the pass device.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a block diagram of a battery charger with active feedback
voltage controller according to a preferred embodiment.
FIG. 2 shows a battery charger with active feedback voltage controller
according to a first preferred embodiment,
FIG. 3 shows a battery charger with active feedback voltage controller
according to a second preferred embodiment.
FIG. 4 shows a battery charger with active feedback voltage controller
according to a third preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A battery charger with active feedback voltage controller includes a pass
device, such as a metal-oxide-semiconductor field-effect transistor
(MOSFET), and implements an active feedback voltage controller to control
a tracking regulator voltage supply and reduce power dissipation in the
pass device. Instead of using an actual battery voltage to control the
tracking regulator directly, an active feedback voltage controller uses a
fraction of the actual battery voltage, yet maintains the charging current
required by a current controller. The fraction varies depending on the
desired charging current as determined by the current controller, and
other factors, such as the ambient temperature sensed by a battery
thermistor, the actual battery voltage, or the battery type. In two
embodiments of the active feedback voltage controller, a gate control line
for a MOSFET pass device is connected to a BJT in the active feedback
voltage controller to reduce the control voltage to the tracking
regulator. In another embodiment of the active feedback voltage
controller, a microprocessor directly controls the tracking regulator
depending on the desired charging current and other factors, such as the
actual battery voltage, the calculated present charging current, or the
ambient temperature sensed by a battery pack thermistor.
The active feedback voltage controller may include hardware, software, or a
combination of hardware and software. The active feedback voltage
controller reduces the control voltage to the tracking regulator which in
turn reduces the power dissipation of the pass device, and it allows for a
smaller pass device to be implemented in the battery charger while
maintaining the desired charging current as determined by the current
controller. With software flexibility, many types of batteries can be
efficiently charged to capacity, including nickel-cadmium (NiCad),
nickel-metal-hydride (NiMH), and lithium-ion (LiIon) batteries, while
reducing power dissipation.
FIG. 1 shows a block diagram of a battery charger 100 with active feedback
voltage controller 130 according to a preferred embodiment. A tracking
regulator 110 provides a supply voltage that is a constant DC offset from
a control voltage on a voltage control feedback line 135. A current
controller 120 calculates a present charging current to a battery pack 150
using a voltage at a supply line 122 and a voltage at a sense line 124 on
either side of a sense resistor 142 (I=(V.sub.supply
-V.sub.sense)/R.sub.sense). There are various alternate methods of
determining the present charging current, including placing the sense
resistor could be placed in another location.
After the present charging current is determined, the current controller
120 establishes a current control value based on the present charging
current and stored charging rates from either a memory in the current
controller itself or from a data storage device 153, such as an EPROM,
associated with a battery pack 150. A data line 121 can send the current
controller 120 additional information on data line 121, such as the
battery type and the battery charging table, from a data storage device,
such as an EPROM 153, usually found in a battery pack 150. The current
controller 120 can also receive ambient temperature information on
temperature line 123 from a thermistor 156, which is built into most
battery packs. The current control value determined from the present
charging current, stored charging rates, and data and temperature
information as available, is used to control the pass device 144, which
charges battery cells 151 in the battery pack 150 through a diode 146. In
this implementation, the current control value is expressed as a voltage
on the current control line 126.
An active feedback voltage controller 130 also receives the current control
value from current control line 126 as well as the actual battery voltage
on battery line 128. Using this information, the active feedback voltage
controller 130 determines a voltage control value and expresses that value
as a voltage on a voltage control feedback line 135 to the tracking
regulator 110. The tracking regulator produces an output voltage that is a
constant positive offset from the voltage on the voltage control feedback
line 135. The active feedback voltage controller 130 is designed to
provide the minimum voltage on the voltage control feedback line 135
needed to obtain the charging current specified by the current controller
120. By using the minimum voltage on the voltage control feedback line
135, the power dissipation of the battery charger 100, and especially the
pass device 144, can be reduced. This reduces the thermal dissipation of
the battery charger and may allow the package of the pass device 144 to be
physically smaller.
FIG. 2 shows a battery charger 200 with active feedback voltage controller
230 according to a first preferred embodiment. In this embodiment, the
active feedback voltage controller 230 is implemented using hardware
components, namely a PNP bipolar junction transistor (BJT) 234 and two
resistors 232, 236. The two resistors 232, 236 compensate for manufacturer
variations in the input impedance of the tracking regulator 210. The
current controller 120 shown in FIG. 1 is implemented using a
microprocessor 220 which calculates the present charging current from a
voltage on a supply line 222, a voltage on a sense line 224, and a known
value of a sense resistor 242. The microprocessor 220 then determines the
desired current control value and uses a current control line 226 to vary
the voltage at the gate of the MOSFET pass device 244 to charge battery
cells 251 in a battery pack 250 through a diode 246.
The calculation of the current control value can include variables for
ambient temperature as determined by a thermistor 256 and transmitted by
temperature line 223 to the microprocessor 220 and battery type and
battery charging table information from an EPROM or other type of data
storage device 253 sent through data line 221.
The current control line 226 is also connected to the base of the BJT 234
through a resistor 232. The value of the resistor 232 is selected to
optimize the entire battery charger 200. The emitter of the BJT 234 is
connected to the battery pack 250 using battery line 228, and the
collector of the BJT 234 is connected to the tracking regulator 210 using
voltage control feedback line 235. As the voltage on the current control
line 226 increases, the BJT 234 decreases the voltage on the voltage
control feedback line 235 relative to the actual battery voltage on
battery line 228. When the voltage on the voltage control feedback line
235 is so low as to cause the microprocessor 220 to increase the voltage
on the current control line (due to a change in the present charging
current), the BJT 234 allows the voltage on the voltage control feedback
line 235 to increase. This control system should settle down to a steady
state that achieves the required charging current as calculated using
sense resistor 242 and minimizes the power dissipation of the MOSFET pass
device 244. During equilibrium, the MOSFET pass device 244 is generally
driven at saturation to decrease power dissipation, and the BJT 234 is
generally driven in linear mode or at saturation.
Under certain circumstances, the battery charger 200 may oscillate due to
the fact that the current controller implemented by the microprocessor 220
and the active feedback voltage controller 230 have different response
times. This oscillation can be remedied using a delay circuit to slow the
response time of the faster control loop, which in this case is the active
feedback voltage controller 230. For a LiIon battery charger, this delay
circuit could be implemented using the thermistor 256 and a variable
capacitor.
FIG. 3 shows a battery charger 300 with active feedback voltage controller
according to a second preferred embodiment. The active feedback voltage
controller 330 is also implemented using hardware components, specifically
an NPN BJT 334 is used in a linear mode to create a variable resistor. The
current controller 120 shown in FIG. 1 is implemented using a
microprocessor 320 which calculates the present charging current from a
voltage on a supply line 322, a voltage on a sense line 324, and a known
value of a sense resistor 342. The microprocessor 320 then determines the
desired current control value and uses a current control line 326 to vary
the voltage at the gate of the MOSFET 344 pass device to charge battery
cells 351 in a battery pack 350 through a diode 346.
The calculation of the current control value can include variables for
ambient temperature as determined by a thermistor 356 and transmitted by
temperature line 323 to the microprocessor 320 and battery type and
battery charging table information from a data storage device 353 as sent
through data line 321.
The current control line 326 is also connected to the base of the BJT 334
through a resistor 332. The value of the resistor 332 is selected to
optimize the entire battery charger 300. The emitter of the BJT 334 is
connected to ground, and the collector of the BJT 334 is connected to the
battery pack 350 through battery line 328 and the tracking regulator 310
through voltage control feedback line 335. When the voltage on the current
control line 326 is high, the charger MOSFET pass device 344 is mostly
off, and it is desirable to lower the voltage on the voltage control
feedback line 335 relative to the actual battery voltage on the battery
line 328. As the voltage on the voltage control feedback line 335 is
lowered, the BJT 334 will begin to turn off. An equilibrium will
eventually be reached between the MOSFET pass device 344 and the BJT 334,
and thus the power dissipation can be minimized. Note that this approach,
however, cannot adapt to major manufacturer variances in the tracking
regulator 210 input impedance.
Under certain circumstances, the battery charger 300 may oscillate due to
the fact that the current controller implemented by the microprocessor 320
and the active feedback voltage controller 330 have different response
times. This oscillation can be remedied using a delay circuit to slow the
response time of the faster control loop, which in this case is the active
feedback voltage controller 330. For a LiIon battery charger, this delay
circuit could be implemented using the thermistor 356 and a variable
capacitor.
FIG. 4 shows a battery charger 400 with active feedback voltage controller
according to a third preferred embodiment. In this approach, both the
charger controller and the active feedback voltage controller are
implemented in a microprocessor 420. This eliminates the potential for
oscillation, because the microprocessor 420 directly controls both the
voltage on the current control line 426 and the voltage on the voltage
control feedback line 435. The microprocessor 420 calculates the present
charging current from a voltage on a supply line 422, a voltage on a sense
line 424, and a known value of a sense resistor 442. The microprocessor
420 then determines the desired current control value and uses a current
control line 426 to vary the voltage at the gate of the MOSFET pass device
444 to charge the battery cells 451 in a battery pack 450 through a diode
446. The current control value can also depend upon information from a
thermistor 456 in the battery pack 450 as conveyed through a temperature
line 423 or information from a data storage device 453, such as an EPROM,
sent through data line 421.
The microprocessor 420 also receives an actual battery voltage on battery
line 428 and determines the proper voltage to put on voltage control
feedback line 435 going to the tracking regulator 410. This voltage is
preferably determined using a mathematical formula; however, a table or
particular parameters can be used to set the voltage on voltage control
feedback line 435. A simple algorithm would stepwise increase the voltage
on the voltage control feedback line 435 if the present charging current
is less than the desired current control value, stepwise decrease the
voltage on the voltage control feedback line 435 if the present charging
current is greater than the current control value, and also stepwise
decrease the voltage on the voltage control line if the present charging
current is approximately equal to the current control value.
Thus, a battery charger with active feedback voltage controller actively
reduces the feedback voltage to a tracking regulator, which in turn
reduces power dissipation in a pass device. While specific components and
functions of the battery charger with active feedback voltage controller
are described above, fewer or additional functions could be employed by
one skilled in the art within the true spirit and scope of the present
invention. The invention should be limited only by the appended claims.
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